At least in newer versions of Excel, you can right click the Sheet tab and choose "Protect Sheet," then follow the instructions. To view the worksheet, you have to right click the tab again and choose "Unprotect Sheet" (which will prompt the pop-up message to enter the password). Looks like lost passwords cannot be retrieved though, so something to keep in mind.

For 2007, it looks like the option might be under the Review tab, at least according to this help file: Protect Files, Sheets and Cells in Excel 2007 - Knowledgebase / Software / Microsoft Office - Brown IT Service Center